(1) Field of the Invention
The present invention relates to a servo-control, as well as a rotor and an aircraft provided with this servo-control.
(2) Description of Related Art
Conventionally, an aircraft comprises piloting members called “maneuvering members” for convenience. The maneuvering members make it possible to control the displacement of the aircraft in space. These maneuvering members can comprise blades of a rotor and in particular a lift rotor of a rotary-wing aircraft, or even rudders or elevators for example.
The control members of the aircraft are driven by members called “control members” for convenience. The control members are linked to the maneuvering members by control chains. For example, control members can comprise an automatic piloting system and/or flight controls maneuvered by a pilot.
An automatic piloting system can include a computer driving at least one cylinder. For example, the computer drives a cylinder with slow action and full authority on the control and/or at least one cylinder with rapid action and with limited authority.
Certain aircrafts have an assistance member for boosting the force exerted by a pilot or by an automatic piloting system. On a helicopter, hydraulic servo-controls are conventionally used for this purpose, each servo-control being driven by the control members. The control members are thus linked to the hydraulic distributors of the servo-controls.
Usually, the servo-controls comprise a cylinder provided with at least one body and one power shaft.
Among the different types of cylinder, a single-body cylinder is provided with a single body in which is displaced a piston borne by the power shaft. The power shaft can comprise one or more pipes.
A multi-body cylinder is provided with a plurality of bodies. Each body houses a piston, the pistons being borne by the power shaft. A double-body cylinder is commonly used in the aeronautical field. When the servo-control is provided with several bodies, these bodies are secured to one another.
Thus, a servo-control cylinder comprises a subassembly provided with one or more bodies depending on the type of cylinder.
Independently of the cylinder type, this subassembly and the power shaft are mobile in translation relative to one another.
For example, the power shaft is articulated at a fixed point in the reference frame of the aircraft, the body subassembly being articulated to a maneuvering member that is mobile in this reference frame. Consequently, each body slides along the power shaft. Such a servo-control is called a “mobile-body” servo-control.
Alternatively, the power shaft is articulated to a mobile maneuvering member, the body subassembly being articulated at a fixed point in the reference frame of the aircraft. Consequently, the power shaft slides along each body. Such a servo-control is called a “fixed-body” servo-control.
Whatever the alternative, the cylinder of the servo-control therefore comprises a mobile member and a member that is substantially immobile to be able to be extended or retracted.
Moreover, each body comprises an outer jacket delimiting an internal space. Consequently, each control piston divides the internal space of the body into a retraction chamber and an extension chamber. The expression “retraction chamber” denotes a chamber provoking the retraction of the servo-control when said chamber is filled by a hydraulic fluid. Conversely, the expression “extension chamber” denotes a chamber provoking the extension of the servo-control when said chamber is filled by a hydraulic fluid.
Such a hydraulic fluid is more simply called fluid hereinbelow, and can be oil for example.
Furthermore, the servo-control comprises a hydraulic distributor for each body.
When a control member requires the displacement of a maneuvering member, an order is then transmitted to the hydraulic distributor of each body. The hydraulic distributor injects the hydraulic fluid into the appropriate hydraulic chamber. Depending on the orders given, the hydraulic distributor therefore injects the hydraulic fluid into the retraction chamber or the extension chamber of a body and thereby induces the retraction or the extension of the servo-control. The hydraulic distributor also allows the evacuation of the fluid from the other chamber.
The intake of a fluid under pressure into one of the chambers of a body generates a pressure difference between the pressures prevailing in the retraction chamber and the extension chamber of a body. This pressure difference tends to displace the body of the cylinder or the power shaft depending on the nature of the servo-control to a position of equilibrium. When the position of equilibrium required of the servo-control is reached, the hydraulic distributor is closed.
To this end, a hydraulic distributor can comprise at least one slide valve that is mobile in a housing. The controls of an aircraft are then arranged to induce a displacement of the slide valve in relation to the housing. Depending on the position of the slide valve in the housing, the slide valve allows or prohibits the circulation of the fluid through the hydraulic distribution between a hydraulic circuit and the cylinder of the servo-control.
A hydraulic distributor can possibly comprise a single slide valve called the “main slide valve” for convenience. Alternatively, a hydraulic distributor can comprise a main slide valve that is mobile in a secondary slide arranged in the housing. In normal conditions, the main slide valve is mobile in relation to the secondary slide valve, this secondary slide valve being immobile in relation to the housing. In case of ceasing of the main slide valve in the secondary slide valve, the main slide valve and the secondary slide valve are displaced together in relation to the housing.
Independently of the variant of the hydraulic distributor, each slide valve can be mobile in translation or in rotation in relation to the housing.
The control members therefore make it possible to control the position of at least one slide valve in the housing, for example to connect a fluid supply orifice of the housing with a chamber of a body of a servo-control, and a fluid evacuation orifice of the housing with the other chamber of this body.
In order for the circulation of the fluid to be interrupted when the cylinder reaches the required position, the position of the slide valve in relation to the housing is interlocked with the position of the cylinder, namely the position of the power shaft in relation to each body of the servo-control. This interlocking can be produced mechanically. According to one example, a connecting rod called a “mimic rod” links the mobile member of the cylinder to the housing, in particular with a fixed-body servo-control. On some servo-controls, the housing of a hydraulic distributor can be fixed to a body.
According to another aspect, the cylinder comprises a mobile member and a fixed member, namely, respectively, the bodies and the power shaft on a mobile-body servo-control or, respectively, the power shaft and the bodies on a fixed-body servo-control. Consequently, the servo-control comprises multiple dynamic seals arranged between the mobile member and the fixed member.
A first dynamic seal can be arranged on each control piston, between the control piston and the jacket of the body. The function of such a dynamic seal is to prevent an undesirable passage of fluid between the retraction chamber and the extension chamber of a body.
A second dynamic seal is also arranged between the power shaft and each body of the servo-control.
However, these can have degraded performance levels due to wear inducing leaks internal and external to the servo-control. These two types of leaks reduce the performance levels of the servo-control and therefore have an impact on the maintenance of this servo-control.
A leak of a second dynamic sealing induces an escape of fluid out of the servo-control. The leak can be detected by a visual inspection and results in the repair of the servo-control.
Conversely, the internal leaks between two chambers of one and the same body cannot be detected visually. These internal leaks result from a degradation of the first dynamic seals.
A so-called “dormant” failure can then occur assuming an undetected leak of a first dynamic seal.
To detect such dormant failures, a manufacturer can provide comprehensive maintenance operations performed at regular time intervals. These maintenance operations involve dismantling the servo-controls from the aircraft or using specific tools that are complex and difficult to implement. These operations therefore have a not-inconsiderable cost. The invention aims to optimize the detection of a leak of a first dynamic seal. On an aircraft that invokes a servo-control with a relatively high frequency of operation, the seals can wear prematurely and incur numerous maintenance operations.
The document FR 3020038 describes a hydraulic system provided with a servo-control. This servo-control comprises a cylinder allowing a leak between each body and the power shaft of the servo-control. Consequently, the hydraulic system comprises an enclosure surrounding the servo-control to collect the fluid leaking out of this servo-control.
The document FR 2433659 describes a hydraulic system provided with a main servo-control. The main servo-control is controlled by a lever via a secondary servo-control.
The documents GB544793, DE102004045011 and FR 2009421 are also known.
The document GB 544793 describes a servo-control provided with a position mimic lever which extends between a power shaft of the servo-control and a hydraulic fluid distribution slide valve. A shaft linked to a double spring cooperates with the position mimic lever.